The present invention relates generally to the art of nuclear medicine. It finds particular application to nuclear imaging techniques and apparatuses employing emission and transmission tomography. Although the present invention is illustrated and described herein primarily in reference to positron emission tomography (PET) and single photon emission computed tomography (SPECT), it will be appreciated that the present invention is also amenable to other noninvasive investigation techniques and other diagnostic modes in which a subject or patient is examined with transmitted radiation.
Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads are placed adjacent to a surface of the subject to monitor and record emitted radiation. For SPECT imaging, collimators are typically placed on the detector heads. For PET imaging, a coincidence detector detects concurrent receipt of a radiation event on two oppositely disposed heads. PET imaging can be performed using a thin collimator or axial filter to minimize stray radiation.
Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject. Such images typically provide functional and metabolic information.
Positron emission tomography (PET) is a branch of nuclear medicine in which a position-emitting radiopharmaceutical, such as 18F-fluorodeoxyglucose (FDG), is introduced into the body of a subject. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays for FDG. The gamma rays are emitted in directions 180xc2x0 apart, i.e., in opposite directions.
A pair of detectors registers the position and energy of the respective gamma rays. Two concurrently received events define a ray which provides information as to the position of the annihilation event and hence the positron source. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct a clinically useful image.
Single photon emission computed tomography (SPECT) is another nuclear imaging technique used to study the radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads are placed closely adjacent to a surface of the subject to monitor and record emitted radiation. Collimators mounted on the heads define the trajectory of radiation that is recorded by the head. In SPECT imaging, the detector head or heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation emission data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems in nuclear imaging techniques such as PET and SPECT is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head(s) distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogenous in terms of radiation attenuation, with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Radiation that is not attenuated is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into a transmission image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission image representation are used to correct for radiation attenuation in the emission data.
Since transmission image data is structural or anatomical in nature; whereas, emission image data is functional or metabolic in nature, it would be desirable to use transmission image data for image localization and/or image registration with a structural image of the same region from another imaging modality. The combination of a functional emission image with a structural transmission image or an image from another imaging modality can provide the diagnostician with insights that could not be obtained with either image alone, thus improving diagnostic accuracy. For example, in the area of oncology, precise positioning of localization of functional images enables a clinician to assess lesion progression and/or treatment effectiveness. Also, such diagnostic studies are used in surgical and/or radiotherapeutic planning, where precise positioning is necessary to minimize the effect on healthy cells surrounding the target cells.
While transmission data has heretofore been largely successful in determining attenuation correction factors for correction of the emission image data, the transmission image data itself has generally been of less than ideal resolution. The coarseness of the images could create uncertainties when localizing the emission image with respect to anatomical features. Also, the quality, and thus diagnostic value, of image registration could be improved with a transmission map of increased quality.
Imaging devices which combine a CT-like device with a gamma camera are known in the art. Typically, the patient is registered with only one of the nuclear and CT devices at a time. Such a combined device is a less than optimal solution to the problem of nuclear medicine image localization due to cost, temporal and spatial registration difficulties, and for logistical reasons. Also, although different modalities are combined in a single system, this type of device retains the conventional approach of addressing separately attenuation correction and precise nuclear medicine image localization.
Transmission image quality can also be increased through increasing the number of counts, i.e., by increasing the source activity, increasing the imaging time, or both. Increasing the source activity, however, has the disadvantage of increasing cost and shielding requirements. Increasing the imaging time is generally undesirable for patient handling reasons. Also, both increasing source activity and increasing imaging time undesirably increase the dose of radiation received by the subject.
It is also known to optimize a collimator for either transmission or emission imaging, however, optimizing for one or the other suffers from the drawback that the two sets of requirements can result in conflicting design parameters. For example, a collimator having a geometry that closely matches the position of the transmission source can increase transmission image quality. However, doing so can impose constraints in terms of geometry and potentially cause truncation of the emission data, known to cause severe artifact in the reconstruction.
Accordingly, the present invention contemplates a new and improved nuclear medicine imaging method and apparatus which overcome the above-referenced problems and others.
In accordance with a first aspect of the present invention, a method of diagnostic imaging with a nuclear camera which includes a rotating gantry on which at least first and second detector heads are mounted, each of the detector heads carrying an offset transmission radiation source transmitting transmission radiation through an examination region to the other detector head is provided. The method comprises injecting a subject to be imaged with a radiopharmaceutical composition generating emission radiation and during an emission imaging phase, detecting emission radiation events from the radiopharmaceutical composition and generating emission data based on the emission radiation events detected. During a transmission imaging phase, transmission radiation is transmitted through a subject to be imaged, the subject attenuating the transmission radiation. Also during the transmission imaging phase, transmission radiation events are detected and transmission data based on the transmission radiation events are generated, wherein each detector head is configured to collect one or both of emission data and transmission data using only a portion of each detector head.
In a further aspect, a gamma camera includes a plurality of detectors for detecting emission radiation emitted from within a subject and transmission radiation which has traversed a subject to be imaged, the subject attenuating the radiation, each detector generating position and energy data. A plurality of transmission radiation sources each transmit transmission radiation through an examination region to a detector. Segment selector circuitry connected with the detectors selectively disables a portion of each detector head during collection of emission data, transmission data, or both. A first electronic storage medium connected with the segment selector circuitry stores transmission data and a second electronic storage medium connected with the segment selector circuitry stores emission data. A first reconstruction processor connected with the first electronic storage medium generates a transmission image representation. A second reconstruction processor connected with the second electronic storage medium generates an emission image representation.
One advantage of the present invention is that it allows optimization for emission imaging while allowing transmission imaging to be performed at a high count rate.
Another advantage of the present invention is that it provides a highly detailed attenuation map for anatomical localization and image registration.
Another advantage is that nonimaging segments of the detector head can be turned off to reduce the count rate load on the detector during transmission imaging.
Another advantage is that emission and transmission data can be acquired concurrently with optimized acquisition strategies for each.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.